Abstract - The purpose of this study is to provide detailed dose estimates to the critical cells in tumor formation from the important radionuclides expected to be prevalent in our environment within the next decade. Two groups of nuclides are to be studied, the alpha emitters which predominate in spent nuclear fuel, namely Pu238,Pu241, Am241, Cm242, Cm244, and the beta emitting fission product Kr85. Estimates are that by 1985, 4 x 10 to the 9th power. Curies of the transuranics will be reprocessed per year, while atmospheric Kr85 levels could be as high as 10 to the 4th power pCi/m3 by the turn of the century. Release of transuranics to the environment can result in human exposures by ingest or inhalation. Soluble forms of these nuclides are bone seekers and critical cells on bone surfaces are irradiate. Inhalation of less soluble particulates irradiates critical cells in the lower lung over long time intervals. Krypton-85 is very soluble in body fat and 13% of body storage fat is located in yellow bone marrow in bone shafts. It is thus capable of producing and important beta dose to bone cells adjacent to bone marrow as well as a dose to critical basal cells in bronchial epithelium upon inhalation. There is no human experience with high levels of these emitters to indicate their absolute toxicity. Whether the exposure is from inhaled or ingested material, the distribution of the transuranic nuclides in the body is quite different from that of radium or radon daughter exposure where human data on tumor induction in bone and lung do exist. A key to evaluating the toxicity of these nuclides as well as Kr85 lies in the estimation of the critical cell rad doses for their disposition in the body and comparison of this dose with those estimated where tumor induction in humans has occurred. In this proposal we show that this type of detailed dose estimate is now possible. All of our dose estimates will be computed from measured energy loss data in tissue-like material. The measurement of beta energy loss will allow unique calculations since these data are not as yet available for accurate spatial dosimetry.